The present invention relates to an electron beam lithography apparatus in which a desired pattern is exposed on a wafer or mask plate by means of a digital scanning of a spot beam.
FIG. 1 shows an arrangement of a conventional digitally scanned electron beam lithography system using a double deflector. An electron beam 1 passes through a beam blanker 2, an aperture 3, a subdeflector 4, and a main deflector 5 and reaches a wafer 6. In the diagram, the region surrounded by a solid line indicates the mainfield on the wafer 6 where the electron beam 1 is deflected by the main deflector 5, while the region surrounded by broken lines in the diagram represents the sub-field where the electron beam is deflected by the subdeflector 4. Namely, the deflection from a sub-field to the next sub-field is performed by means of the main deflector 5, while the deflection in a single sub-field is carried out by means of the subdeflector 4. A circuit to control the foregoing electron optical column is shown on the left side part of this diagram. Pattern data and calibration data of deflection distortion are respectively sent from a control computer 15 to a pattern buffer memory 7 and a correction circuit 8. One of the outputs of the circuit 8 serves to designate the center of the sub-field in the field by correcting the deflection error and makes a main deflection D/A converter 9 operative.
In a pattern-dividing circuit 10, a pattern is divided into small regions (dot fields). The portion inside the small region is divided into dots by a dot dividing circuit 11. A correction circuit 12 for sub-deflection corrects the deflection error for every dot by use of correction coefficients at every sub-field which are sent from the correction circuit 8. Outputs of the correction circuit 12 are inputted to a sub-deflection D/A converter 13 and a blanking amplifier 14.
FIG. 2 shows an example of the state in that a single pattern 23 is exposed in the foregoing system arrangement. An enlarged diagram of one sub-field 22 in a field 21 is shown on the right side part of FIG. 2. The trapezoid pattern 23 in one sub-field 22 in the field 21 is divided into seven small regions (dot fields) corresponding to the hatched portion in the diagram. One region 24 of the dot fields is digitally exposed by the spot beam 1 on a dot unit basis.
In this arrangement, the numbers of bits of the D/A converters 9 and 13 which respectively determine the field and sub-field are ordinarily set to values of about eighteen and twelve bits. Assuming that the minimum unit (Least Significant Bit i.e. LSB) of the digital scanning is 0.01 .mu.m, those fields become 2.5 mm square field and 40 .mu.m square sub-field. In this case, the dot exposure of the dot field 24 is performed by the sub-deflection D/A converter 13.
On the other hand, there is generally a contradictory relation between the settling time of the D/A converter (time until the output of the D/A converter reaches a value within an allowable range of the final value after data changed) and the resolution. The settling time becomes long as the resolution becomes high. For example, the settling time is about 100 nsec in the case of the 12-bit D/A converter, while it is about 10 nsec in the case of the 8-bit D/A converter. Further, in such a conventional arrangement, the deflection error correction is performed for every dot and the beam blanking operation is carried out. Therefore, it takes an additional processing time for correction and it is also difficult to adjust the ON-OFF timing of the beam.
On the other hand, a current density of the spot beam, 500 A/cm.sup.2 can be easily obtained from the conventional technology. Assuming that a resist sensitivity is below 5 .mu.c/cm.sup.2, the exposed time per one dot may be below 10 nsec. Therefore, the exposure in such a conventional system is ordinarily determined by the settling time of the D/A converter rather than the resist exposed time. In other words, since it takes a time of about 100 nsec for the spot beam to be digitally moved to the adjacent point, the current density of the electron beam has to be reduced to perform the exposure even if high current density is obtained.